A treasure trove has been discovered in a secret room hidden underground in Christchurch for at least a century.

Archaeologists uncovered a small underground room in the foundations of a 1970s office block demolished in Madras St in the city centre.

When the secret room was opened in March, 1600 treasures from the 19th century were discovered, including a ladies fob watch, a brooch, mustard pots, cutlery, bottles and china.

Underground Overground Archaeology director Katharine Watson said the find was "amazing and surprising".

"It was under a multi-storey office building so we didn't expect to find a thing. You don't expect much to survive construction of those buildings," she said.

"It was chock-full of artefacts. It was quite incredible. There was a tiny wee fob watch. That is quite unusual to find."

Watson said it was unclear how the underground room had been left for so long. "The only thing we could think of was the original house on that site must have been cleared out and everything thrown into a basement and buried and forgotten about."

The 1600 artefacts were discovered in a brick basement about four metres long and nearly three metres wide.

The basement would have been under a house built after 1877 and demolished by 1916.

The haul included over 800 ceramic fragments, including a pot of ointment for gout and rheumatism, a French mustard bottle, and about 360 glass items, including a bottle for a cocaine-based anaesthetic product used in dental procedures from the late 1880s.

A boot and a shoe were also uncovered in the secret room.

The items were washed and catalogued before being stored by the New Zealand Historic Places Trust. Landowners have the right to claim any items found on their property.

Watson said archaeologists have been busy in Christchurch since the earthquakes, recording finds under demolished buildings.

Watson's consultancy has discovered 19th century beer bottles under former hotels and a stone basement full of clay pipes under the Fisher's building on the corner of High and Hereford streets.